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Infectious diseases will be introduced under the types of organisms thatcause them from viruses to bacteria and parasites.
Viruses:Viruses can have either DNA or ribonucleic acid (RNA) as the genetic material.Under the Baltimore system of classification of viruses, they are assigned to one ofseven groups based on the form of the DNA or RNA and not based on the diseases they cause. Viroids are smaller than viruses and are known to be plant pathogens.
Rabies:Rabies is a viral disease of many mammalian species, including bats, dogs, cats,foxes, raccoons, skunks, and people. It is spread chiefly by animal bites. Rabiesis almost always fatal in people and most animals. Rabies vaccination is requiredlegally for dogs and cats in Nigeria and mostcountries of the world. People who have a highrisk of animal bites, such as veterinarians, receive prophylactic vaccination againstrabies. If someone is bitten by an animal suspected of having rabies (being rabid),a series of vaccinations are used. Orally active vaccines at baiting stations are usedto control rabies in wildlife.
Foot-and-Mouth Disease: Foot-and-mouth disease, also known as hoof-and-mouth, is a highly contagiousviral disease of cattle, pigs, and sheep. The disease is caused by a RNA virus, thefoot-and-mouth disease virus. North America has been foot-and-mouth diseasefree since 1954, with the last reported case in the United States in 1929, while Africa and many tropical countries remain endemic. There was a significant outbreak in the United Kingdom in 2001, with 7 million animals culled.This spread to Western Europe. Vaccines against foot-and-mouth disease virus areonly used in countries where foot-foot-and-mouth disease is found because vaccinationprevents exports and does not allow surveillance for the presence of the disease.
Avian Influenza: Avian influenza is caused by one of a series of viruses. The disease not only affects poultry but also can infect both game and wild birds. The latter is particularly a problem because they fly around and can spread the disease locally or, forthose that migrate, spread the disease into different regions and countries. Avian influenza viruses are classified on two bases:
1. The degree of pathogenicity, that is, the ability of the virus to cause disease.The virus can have low pathogenicity or high pathogenicity. Infection with a low-pathogenicity avian influenza virus will cause little overt signs of disease.However, the low-pathogenicity avian influenza virus can mutate into ahigh-pathogenicity virus, with serious consequences.
2. The presence of two glycoproteins on the surface of the virus:
a. Hemagglutinin proteins (H). There are 16 (H1–H16).
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b. Neuraminidase proteins (N). There are nine (N1–N9).
A particularly serious form of the avian influenza virus is the H5N1 virus becauseit is highly contagious among birds, and the disease is frequently fatal to them. Moreover,there are reports of infection by people with the H5N1 virus. In people, pathogenic viruses cause influenza. It is thought that avian influenzaviruses mutate into forms that are pathogenic to people and/or domestic animals.
Avian influenza type A virus may be transmitted from animals to humans in twomain ways:
directly from birds (live or dressed) from avian virus–contaminatedenvironments to people, or through an intermediate host, such as a pig. Transferof the virus to pigs and horses may be from people to domestic animals. The human influenza viruses can be categorized as A, B, and C types. The type A viruses are further classified by subtype based on surface proteins—
hemagglutinin(H) and neuraminidase (N). Subtypes of the virus that have caused influenzain mammals include H3N2, H2N2, H1N1, and H1N2 in humans; H1N1 andH3N2 in pigs;
H7N7 and H3N8 in horses; and H3N8 in dogs.
Bacteria:Bacteria are a simple microscopic form of life. They can produce all the proteinsneeded for life. They use DNA as their genetic material. They are called prokaryotesbecause they do not contain a nucleus and membrane-bound organelles.
Incontrast, eukaryotes (animals, plants, fungi, and protozoa) do contain a nucleusor cellular organelles.The classification ofbacteria is domain,Bacteria; and otherdomains, Eukarya(containing the Animalia,Fungi, Plantae, andProtista kingdoms) andArchaea.
Bacterial diseases include mastitis, the secondary infection of shipping fever incattle, Borreliaburgdorferi, causing Lyme disease in dogs, cats, and people, andbubonic plague.
Many food-borne diseases are caused by bacteria such as Campylobacterjejuniand Campylobacter coli, Escherichia coli, salmonellosis caused by Salmonella bacteria,Listeriaand tuberculosis
Mastitis:Mastitis is the number one disease of dairy cattle. It resultsin much-reduced milk production, the loss of milk not saleable with a high concentrationof somatic cells (leukocytes), and long-term damage to the mammarygland. It is caused by the invasion of the mammary gland by pathogens, includingvarious species of Streptococcus, Staphylococcus, and Mycoplasma bacteria.The innate immune response plays a significant rolein how the mammary gland deals with pathogenic bacteria. The teat canal is a barrierpreventing pathogens from entering the mammary gland. Between milkingand during the dry period, the teat canal is sealed by keratin, which is a plug. This is derived from the stratified epithelial lining of the canal. Inflammation is part ofthe innate immune, and mastitis is inflammation of the mammary gland.Mastitis can be either of the following: clinical or overt (readily seen by observation),or subclinical.
Tuberculosis: Tuberculosis is a chronic disease of many animal species and poultry caused by bacteria of the genusMycobacterium. It is characterized by development of tubercles in the
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organs of most species. Bovinetuberculosis is caused by Mycobacterium bovis. It is a significant zoonotic disease.An infected animal is the main source of transmission. The organisms are excreted in theexhaled air and in all secretions and excretions. Inhalation is the chief mode of entry and for calvesinfected milk is an important source of infection. When infection has occurred tuberculosis may spread:
a) By primary complex (lesion at point of entry and the local lymph node) b) By dissemination fromprimary complex.
In sick animals the condition manifests as;
1. Low grade fever
2. Chronic intermittent hacking cough and associated pneumonia 3. Difficult breathing
4. Weakness and loss of appetite 5. Emaciation
6. Swollen superficial body lymph nodes
Mycobacteria invade cattle by respiratory (90 – 95 %) and oral routes (5–10 %).
Congenitalinfection in the bovine fetus occurs from an infected dam. Tuberculosis lesions can be classified as acute miliary, nodular lesions and chronic organ tuberculosis. Young calves are infected by ingestion of contaminated milk. The incidence of human tuberculosis caused by Mycobacterium bovis has markedly dropped with the pasteurization of milk. It also has dropped in areas where programs of tuberculosis eradication are in place. Man is however susceptible to the bovine type. In cattle, lesions of tuberculosis caused by the avian type are commonly found in the mesenteric lymph nodes. Tuberculosis in small ruminants is rare. In pigs the disease may be caused by the bovine and avian types.
Salmonellosis: Salmonellosis is a disease which occurs in all animals and humans. In animals, salmonellosis ischaracterized clinically by one of three syndromes:
a) Peracute septicemic form:
b) Acute enteritis c) Chronic enteritis.
The young, old, debilitated and stressed animals are at greater risk. More than 200 antigenically differentserotypes of Salmonella have been identified and all of these possess pathogenic potential. The mostfrequently identified serotypes of the organisms which cause the disease in cattle are S. typhimurium, S.dublin, S. muenster and S. newport. Salmonellosis in stressed animals is frequently associated withinadequate diet, irregular feeding, water deprivation, overcrowding, parasitism, weather extremes,pregnancy, parturition, inter-current diseases etc. The calving complications which may predispose thedisease include abortion or early termination of pregnancy, retained placenta, endometritis and post-parturient metabolic condition
It is transmitted through the ingestion of feed that have been contaminated by the faeces of infected animals, bydrinking water in stagnant ponds and by the carrier animals. In housed
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animals, transmission is viacontaminated feedstuff containing improperly sterilized animal by-products such as bone and meat mealand fish meal. Casual workers, infected clothing and utensils, transportation trucks and birds may transmitthe disease to the farm. Active carrier animals shed Salmonella organisms intermittently and withoutobvious stress factors. Latent carriers with stress factors are also identified in the transmission ofsalmonellosis.Human infection is transmitted via contaminated water, raw milk and meat. Compared to bovines, pigs and poultry are more significant sources of infection in humans.
Leptospirosis: This is the disease caused by clinical infectionwith any one of the many serovars of the bacteriumLeptospirainterrogans. Each serovar of the bacterium ismaintained in nature by non-clinical persistent infection ofone or more wild or domestic mammals. These mammals
are the maintenance hosts.Leptospirosis as a clinical disease occurs when mammals ofother susceptible species, such as humans, becomeinfected. The disease is associated with septicemia,hemolytic anemia, hepatitis, nephritis, jaundice,abortion and still births. The bacteria persist in the kidneysof the maintenance hosts, are shed (excreted) in the urine,and can survive for some time in aquatic and moistenvironments. Infection can occur through the followingways:
– ingestion of contaminated water
– handling or ingesting infected milk or tissues – transplacental invasion
– sexual contact – social grooming.
In humans, farm and abattoir workers, hunters andtrappers, wildlife handlers and zoo-keepers havetraditionally been the high-risk groups. Epidemicoutbreaks may be associated with periods of high rainfall, particularly in habitats with poor drainage and a high density of carrier animals. Thus, while leptospirosis has been an acknowledgedzoonosis for many ears, recently it appears to be gainingnew importance as a public health threat.
Brucellosis:Several species of Brucellainfect animals. Infection oflivestock by any of these species, whether or not theinfection results in disease, may cause the animals to test‗positive‘
in standard screening tests used to identify andeliminate infected domestic animals or herds.Brucellaabortusand B. melitensisare the species mostregularly transmitted between wild and domesticungulates, and are most frequently associated with theconflicting needs of agriculture, and the risk ofhuman disease. Each species can cause significant diseasein livestock (B. abortusin cattle and B. melitensisin sheepand goats), and both can cause serious disease in humans.Human health risks are generally associated with thehandling or consumption of infected animals or products.
These Brucellaspecies also pose potential health risks topeople who handle or consume infected animal products.Indeed, at least 91 species of wild mammals, from ninedifferent orders, demonstrate some evidence of infectionwith one or more species of
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Brucella.Infections can result in abortion,lameness and sterility on the individual level, but manyinfections appear to be sub-clinical.
Parasites and disease: A parasite is an organism that lives in close proximity to an animal and completelydepends on it. Parasites receive their nutrition from the host, using thehost‘s blood or absorbing nutrients in the host‘s intestine. Although the parasitesrarely kill the host, they do inflict significant harm. Parasites adversely affect thequality of life for companion animals and livestock; they also reduce production efficiency of livestock and may result in the death of an animal.Internal parasites include protozoa, roundworms or nematodes, flatworms ortrematodes, cestodes or tapeworms, and some flies such as a botfly.Ecto-parasites can be either insects or arachnids (ticks and mites, bugs, fleas, flies,lice, or mosquitos).
Protozoa: Protozoa are single-celled eukaryotes with a nucleus and intracellular organelles.
They impact the livestock and companion animals as parasites, as zoonotic diseases, and by symbiotic ciliates participating in the fermentation in the rumen.For example, trypanosomes are protozoa with flagella that can cause serious diseases in animals and people. Examples include the following:
• The trypanosome T. bruceicauses African trypanosomiasis (or sleeping sickness) in humans and nagana in cattle in Africa. The vector is the tsetse fly. The presence of the tsetse belt effectively closes some 10 million km2 in Africa to efficient cattle production.
• T. cruzicauses Chagas‘ disease in Central and South America. Its vector is the blood-sucking bug Rhodniusprolixus.
• Trypanosomaequiperdumare sexually transmitted in horses.
Coccidia are intracellular parasites of the intestinal cells. The following are examples of coccidian and coccidial diseases:
• Cryptosporidium is a widespread zoonosis.
• Coccidiosis in dogs and cats is due to coccidia of the genus Isospora.
• Toxoplasmosis is due to ingestion of protozoan parasites, Toxoplasma gondii, in raw or undercooked meat, or through contact with an infected cat.
• Bovine coccidiosis.
• Coccidiosis in poultry due to Eimeriatenella. Without coccidiostats, production of poultry would be severely impacted with significant mortality (deaths), morbidity (sickness), and reduced growth and poorer feed conversion efficiency. A number of coccidiostat are available commercially.
Giardia intestinalisis a common parasite of cats. It also infects dogs, cattle, sheep, and people. Protozoan parasites Theileriaannulataand Theileriaparvainfect cattle in sub-Saharan Africa. This greatly impacts the potential development of theindustry. Ticks are the vector.
Another protozoan parasite is Histomonasmeleagridis,which causes a disease called blackhead in chickens and turkeys.
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Roundworms or nematodes: Roundworms are from the phylum Nematoda, and include both parasitic and free-living species. Examples of roundworm parasites are the following:
• Intestinal roundworms (ascarids):
• Dogs, with two species of roundworms (Toxocaracanisand Toxascarisleonina)
• Cats, with two species of roundworms (Toxocaracatiand T. leonina)
• Pigs, with the large roundworm (Ascarissuum)
• Horses, with the large roundworm in horses (Parascarisequorum)
• Chickens and turkeys (Ascaridiagalli)
• Strongyles; for example, in horses, there are large strongyles (also known as red worms or bloodworms) and small strongyles.
• Pinworms found in, for instance, horses and people.
• Heartworm (Dirofilariaimmitis), which is found in dogs and cats.
• Trichinella. spiralis which ispresent in pig muscle causes trichinosis in people who eat pork.
• Hookworms, which are small nematodes that attach to the gastric mucosa.There are hookworm species that infect cats, dogs, and people, with the potential for anemia
Flatworms: An example of a flatworm is the sheep liver fluke Fasciola hepatica. Tapeworms live in the intestines of their primary host. The scolex or head of the tapeworm embeds itself in the upper small intestine using hooks and suckers. From the head emerges a chain of proglottids or independent segments, each about the size of a grain of rice. Additional proglottids are being continuallyproduced, and, therefore, with time, they move down the gastrointestinal tractto be eliminated with the feces. The tapeworm has no digestive system.
The proglottidsjust absorb nutrients that otherwise would go to the host. When mature,each proglottids will have both a male and female reproductive system, and whena terminal proglottid(s) breaks off the chain, each will contain about 20 eggs.
The common tapeworm of dogs and cats (Dipylidiumcaninum) is up to 25 inches (62 cm) long. The intermediary host is the flea, with dogs and cats eating the fleasduring grooming.
The eggs hatch in the fleas and develop into infectious larvae.Signs that a dog or cat has a tapeworm infestation include the presence of proglottids,white large seed-like structures with a squirming motion, in the fecesor around the anus. In addition, a dog may exhibit a scooting behavior if the anusis irritated.
The most important pathogenic tapeworms to humans are the porktapeworm (Tenia solium) and beef tapeworm (Tenia saginata). In these cases, peopleare the primary host, and pigs and cattle are, respectively, the intermediary hosts (Figure 22).
44 Fig. 22: Life cycle of the beef and pork tape worms
The beef tapeworm causes cysticercosis in cattle. Three species of tapeworms infect the horse: Anoplocephalaperfoliata, which isthe most common tapeworm; Anoplocephala magna; and Paranoplocephalamamillana.Horses with infections are not at their healthiest.
Diagnosis is difficult, withsubclinical infections causing damage to the gastrointestinal tract and reducingnutrient absorption, but not causing visible disease.
External parasites (Ecto-parasites):Ecto-parasites can be either insects or arachnids (ticks and mites, bugs, fleas, flies, lice, or mosquitos).Ticks and mites live on the skin of animals and exist by sucking blood, but some use dead skin. Like spiders, these are arachnids. Other examples of ticks and mites include the following:
• Dog tick (Dermacentorvariabilis).
• Deer tick (Ixodesscapularis), which is the vector for Lyme disease caused by some spirochetal bacteria.
• Lone Star tick (Amblyommaamericanum).
• The mite, Sarcoptesscabiei, infecting dogs, cats, pigs, and people. In dogs, this sarcoptic mite causes mange, also known as sarcoptic mange or canine scabies. Another mite that infects dogs is Demodexcanis. This may result in skin disease.
• Poultry mites – the Northern Fowl Mite (Ornithonyssussylviarum) and the chicken mite (or Red Roost Mite).
Examples of parasitic insects include the following:
• Lice (order, Phthiraptera) (nomenclature, singular louse; a louse egg is a nit).
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Lice can be either blood sucking or chewers on the skin. An example is poultry lice, Menacanthusstramineus.
• Fleas (order, Siphonaptera), such as the cat flea (Ctenocephalidesfelis) and dog flea (Ctenocephalidescanis).
• Blood-sucking bugs (order, Hemiptera), e.g., R. prolixus.
• Blood-sucking true flies, e.g., mosquitoes and tsetse flies.
Non-infectious diseases
Non-infectious diseases include diseases in which there are nutritional, metabolic,stress-related, and genetic causes. Examples of such diseases are milk fever;founder or laminitis (inflammation and edema of the laminae of the hoof) in horses,which is associated with dietary carbohydrate; ascites (accumulation of fluid in thebody cavity) in meat-type chickens;
and leg weakness/tibial dyschondroplasia inmeat-type poultry. Stress diseases include heat stress. In cats and dogs, obesity,leading for instance to diabetes mellitus, is a growing problem. In addition tononinfectious diseases, there are also bone fractures and other injuries.
Milk fever: Milk fever is a metabolic disease in cattle occurring at the transition between late pregnancy and early lactation. In milk fever, cattle have hypocalcemia or reducedconcentrations of calcium in the blood. This can be either subclinical, in whichthere is hypocalcemia (calcium <2 mmol/L) but with no obvious clinical signsof the disease (but reduced muscle function and depressed milk production), or clinical, in which the hypocalcemia is so severe (calcium <1.4 mmol/L)that nerve and muscle functioning is greatly perturbed and the cow collapses(a ―downer‖ cow). The mortality with milk fever is about 5%, and there is a3.5-year reduction in the life of affected cattle.
Ascites:Ascites is a metabolic disease in poultry, with fluid accumulation in the body cavity, an inability to supply tissues with sufficient oxygen, and a flaccid but enlarged heart.It is estimated that 8% of the mortality of meat-type chickens is due to ascites.
Edema:Edema is the accumulation of excess fluid in the intercellular (interstitial) tissue compartments, includingbody cavities.There are two types of edema: inflammatory (exudate) and non-inflammatory (transudate) edema. Inflammatory edema shows yellow, white or greenish clear or cloudy fluid in the area of inflammation. Non-inflammatoryedema is an accumulation of fluid in subcutaneous tissue, submucosae, lungs and brain.
Localized edema is noted after:
a. The swelling of a leg of a cow in prolonged decubitus. This swelling is caused by obstruction of thevenous outflow
b. Interference with the lymph circulation of an organ or area by proliferation of tumours in or aroundbile ducts.
c. Inflammation or an allergic reaction
Systemic or generalized edema may occur secondary to congestive heart failure or is caused by lowprotein levels in the blood. The latter may be associated with: severe malnutrition, severe amyloidosis of the kidney, gastrointestinal parasitic infestation, chronic liver disease,
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damage to the vascular endothelium by toxins and infectious agents. Anasarca is a form of edema of the subcutaneous tissues. Ascites is an accumulation of fluid in theperitoneal cavity.
Hydrothorax is an accumulation of fluid in the pleural cavity.
Bruises:Bruises are frequently found on food producing animals andpoultry. In cattle bruises caused by transportation or handling are commonly found in the hip, chest andshoulder areas;
in pigs within the ham and in sheep in the hind leg. Bruises and hemorrhages in the hipjoint are caused by rough handling of animals during shackling. Bruises in poultry can be localized orgeneralized and are frequently associated with bone fractures or ruptured ligament tendons.
Tumours: A tumour is an abnormal mass of tissue which grows without control and uncoordinated with the tissue ororgans of origin or those nearby. Its presence if often cumbersome to the tissue or organ it arose either bypressure or by replacement of normal functional tissue. Tumour cells resemble healthy cells howeverserve no useful purpose. The term tumour in current medical lexicon is presently limited to neoplasticgrowths.Tumours are usually divided according to tissue of origin i.e. epithelial, mesenchymal (connective tissue), haemopoietic, nervous etc. Tumour behavioral classification include their mode of growth and the degreeof invasiveness. Slow growing non-invasive circumscribed tumours are considered benign and fastgrowing, infiltrative and frequently metastatic are malignant tumours. The spread of neoplasm is by directexpansion and infiltration, via lymphatics and blood circulation and by implantation.